Forget USB MIDI interfaces, even the best interface will not be able to manage this amount of gear with a correct timming.
Your problem has two sides, each one of equal importance:
1) The new computers that relay in a PCIe architechture (it doesent matter if they are PC or Mac)
2) The MIDI interface itself.
With the first problem there is not a satisfactory repply, the newer computers will give you a noticeable worst timming than your Mac G3, so dont replace the computer until it would be strictly necessary.
For the second problem, the replacement interface, there are new solutions that are not widly tested but the look that would be possible solutions.
This interfaces are generally called LAN interfaces an are connected to the computer via the Ethernet port, (RJ45 connector).
The Ethernet base protocol is widly more efficient than the USB and supports better asynchronic comunication of MIDI.
There is a condition that´s necessary to consider: The Ethernet bus can be connected, in general, directly to the south bridge of the chipset, that´s the best option, or it is usually connected to a PCIe bus, many times sharing the bus with other peripherials of the motherboard.
In any of both cases for a stable and gapless comunication it is very important that the LAN Ethernet connection DO NOT SHARE its direct connector with other device (like USB or SATA, etc) and/or not have its PCIe bus shared with other devices (like USB or SATA, etc). So you will need to search a motherboard that acomplish this requirements
This is the only way to be sure that the hardware will have a REAL IRQ (not a virtual a REAL like in the old machines) only for its own usage not shared with anything else.
The advantage of LAN Ethernet interfaces is that you can reduce the ASIO or Audio Units buffers to really very small sizes, which implies low latencies and better MIDI timestamp.
The two marks of LAN interfaces that I know run under different and uncompatible enviroments and both support Mac and PC and 64bit drivers.
One works with Copperlan (
http://www.copperlan.org) that is free, is the one I use and works incredibly well. It is used by the Alyseum AL-88 and AL-88c
http://www.alyseum.com
The other works with rtpMIDI from Tobias Erichsen, it is easy to find it in the web, and it is used by the iConnectivity interfaces Mio 4 and Mio 10.